Strictly, 姨奶奶 is your paternal grandmother's sister. Your maternal grandmother's sister is 姨姥姥 in the north, or 姨婆, though many city families now say 姨奶奶 for both. Your paternal grandfather's sister is 姑奶奶.
history
In big households of the past, servants called the master's concubine 姨奶奶; the general word for a concubine was 姨太太. Older novels still use it this way.
